Zonal isolation, defined as sealing of the wellbore against unwanted movement of fluids, is a significant challenge for safely extracting oil and gas from offshore and deepwater reservoirs, and has become one of the most important environmental concerns for the oilfield industry (Boesch, D. “Deepwater drilling remains a risky business”. Nature 484:289, 2012). In the worst-case scenario, oil or gas moves vertically upward from the reservoir along the borehole and escapes into the surrounding land or seabed. In particular, zonal isolation is a significant challenge for producing shale gas reservoirs due to the stresses generated in the wellbore by hydraulic fracturing, which can damage the isolation material.
The standard approach to zonal isolation is to pump cement slurry into the annular space between the formation and production casing and allow it to harden in place. Ensuring long-term zonal isolation requires a durable material with low permeability and reasonably high compressive strength that completely fills the annular space. Another approach to sealing, widely used in the automotive and aerospace industries and increasingly used in the oilfield, is to utilize gaskets, O-rings and packers made of elastomers. For these applications, the seal provides a localized block against fluid flow. The maximum differential pressure that the seal can sustain is primarily determined by the contact pressure between the seal material and its confinement, and by the modulus of the seal material (Kim et al, “Approximation of contact stress for a compressed and laterally one side restrained O-ring. Eng Fail Anal 14: 1680-92, 2007). In some cases, the seal relies on its swelling. The ability to seal is crucial to the functioning of diverse components (from a refrigerator or a car to a space shuttle) and sealing failures are responsible for a significant fraction of mechanical breakdowns, leading sometimes to catastrophic results.
Once a zonal isolation material, example, cement or rubber is placed in an annular gap between the formation and a casing/liner hanger or tubing and the casing, the material is expected to exert a load against the confinement (either the formation or the casing/liner hanger). If no compressive load is present, any movement of the confining medium (casing undergoing positive hoop strain for example) or the base tube (tubing undergoing negative hoop strain for example), may cause the sealing to be lost. This type of movement happens regularly in oil wells where the temperature and pressure fluctuate due to pumping and producing cycles. It is important therefore to be able to predict seal performance and potential failure of materials which are used in zonal isolation.